Preserve comments

This appears, at least with the limited testing I've done, to remove comments from my template files. This wouldn't be such a big deal, but there is one specific use case where it is, which is making the `user-data` file for `cloud-init` images. The first line of the file, must be `#cloud-config`. I am not sure if this is a packer requirement, or cloud init or AutoInstall, but it needs to be there none the less. Is it possible to ensure that comments are left in the output file?